Electrical discharge lamps are widely used in various forms, such as fluorescent lights, neon lights, mercury vapor lights and sodium vapor lights. These and many other types of electrical discharge lamps are known and possible using technology which began in the 1800's when many scientists experimented with electrical discharge lamps.
Electrical discharge lamps are characterized by an envelope of glass or other transparent material which encloses a volume of appropriate gas. The enclosed gas can be of a variety of types and combinations which are capable of being ionized to allow electrical current to flow therethrough. Examples of suitable gases employed in electrical discharge lamps include air, neon and argon. These gases are often combined with small quantities of suitable metals and other materials which improve the ionization or light emissive properties of the lamp. Examples of metals commonly used in discharge lamps are sodium and mercury, which vaporize as a result of the heat generated by the lamps. Discharge lamps are also manufactured using combinations of gases such as neon and argon with metal halides such as mercury iodide and sodium iodide.
The variety of gases and added materials used in discharge lamps have widely varying voltage requirements for initiating ionization. The voltage or potential required across the electrodes before ionization will occur depends upon the gas type, internal pressure of the gas, gas temperature, and electrode spacing. After the gas within a discharge lamp becomes ionized, current flows more readily because of the increased number and density of available charge carriers. The increased number of charge carriers greatly reduces the resistance across the electrodes as compared to the starting resistance required when initiating ionization. This decrease in the electrical resistance across the lamp electrodes requires that some form of current limiting device be used in conjunction with the discharge lamp to control the flow of current and prevent the destructive amounts of heat which would be caused thereby. Current control is also desired to reduce power consumption and optimize the illumination output of the lamp. This current limiting function for discharge lamps has typically been performed by an electrical device termed a ballast.
Prior art discharge lamp ballasts have typically used a transformer or other induction coil between the source of electricity and the discharge lamp in order to limit current flow through the lamp. Such transformer ballasts have also often been used to boost the starting voltage to the lamp. Such prior art inductive ballasts suffer from a number of disadvantages. Transformers are relatively costly to manufacture and are also relatively large and heavy. This increases the total cost of the discharge lamp and further requires that relatively strong standards, poles, overhanging arms and other supporting structures be employed. Increased size and strength for foundations and other structural members must also accordingly be provided.
It has also not been practical to remotely mount transformer ballasts at the base of a light pole or otherwise in a remote location because of the relatively high starting or ionization voltage required. Supplying such starting potential has been difficult or impossible to attain when lengths of wire greater than 25-30 feet have been used because of line losses and voltage decreases occurring due to capacitance developed across the supply wiring. Accordingly, it has been standard practice to mount the heavy, bulky transformers immediately adjacent the lamp.
The close mounting of inductive ballasts to discharge lamps typically causes very significant increases in installation and maintenance costs. Installation costs are increased because of the increased size and structural capability which must be provided in any light fixture and supporting structure. Placement of such heavy ballasts in street lighting and other applications also usually entail an overhanging configuration in the added weight of the ballasts which further increase the demands placed upon the supporting poles and other structural elements. Since these poles and other supporting structures are often tall, slender, and free standing, the incremental weight of the inductive ballasts require a disproportionately large amount of the installation costs. Further aggravating these basic structural problems are the effects of wind upon light standards. The large size of the ballasts and associated hoods are more easily displaced by wind forces striking the units atop typically slender light standards, thus displacing the load further off center and intensifying the structural loading problem associated with the weight of the ballasts.
Inductive ballasts must also be shielded from the wind and weather thus requiring additional expense for protective hoods or other coverings. Such protective hoods are relatively large thus increasing the wind loading and weight placed upon the structure which still further increases the costs of manufacturing and installation.
The installation costs of discharge lamp lighting is further increased when transformer ballasts are used because of the relatively high costs of crating, shipping and handling the heavy and bulky transformer. Manufacture of such transformer ballasts also requires relatively large scale heavy industry in order to produce economically. The materials and costs of constructing inductive ballasts are accordingly high.
Maintenance of transformer ballasts has also proven to be costly and difficult. Transformer ballasts produce substantial amounts of heat which tend to deteriorate the coil winding insulation thus leading to short circuiting of the coils and replacement of the ballast. Since the transformer ballasts cannot be conveniently mounted in remote locations from the lamp, this often requires cranes in order to remove and replace deficient ballasts. This accordingly increases maintenance costs.
Vibration produced by transformer ballasts may also cause fluctuating or cyclical loading on the light fixture supporting structures which requires increased strength, or in some cases premature failure, resulting damage and maintenance costs. The expected service life of transformer ballasts is also sufficiently short for the above and other reasons so that maintenance must be performed on a regular basis where numerous units are in service.
Prior art transformer ballasts also suffer from a tendency to vibrate at 60 Hz and several upper harmonies thereof thus producing very noticeable and often irritating noise. This noise has restricted most types of discharge lamps to exterior uses only. Fluorescent type discharge lamps are widely used in interior applications because they do not produce as much noise as other more efficient types of discharge lamps which are noisier. Considering the widespread use of fluorescent lamps, this results in tremendous increased power costs for using fluorescent type lamps versus sodium vapor and other more efficient lamps.
Prior art inductive ballasts are also disadvantageous in providing an inductive power factor component. Power companies typically experience excess inductive as compared to capacitive reactive power factor components, thus requiring installation of power factor correcting equipment such as large banks of capacitors. Such equipment is expensive and accordingly increases the cost of power to the consumer. Thus there is a need for discharge lamp ballasts which produce a capacitive power factor which can be used to offset power consumed by inductive devices such as electric motors.
The prior art includes U.S. Pat. No. 4,337,417 to J. Johnson for a starting and operating apparatus for high-pressure sodium lamps. The Johnson apparatus uses an inductive ballast to control current flow. A dual capacitor arrangement is provided with current through blocking diodes to charge one of the capacitors to approximately double peak line voltage. A zener diode is used to gate a silicon controlled rectifier into a closed condition so that the high voltage charge on the highly charged capacitor flows through the inductor and provides a voltage peak used to start an associated high pressure sodium discharge lamp. The Johnson capacitor arrangement is used to start but does not control current flow through the lamp during normal operation.
U.S. Pat. No. 4,406,976 to Wisbey et al discloses a discharge lamp ballast circuit having an inductor and capacitor in series with two discharge lamps. A bidirectional gated switching device is used across the discharge lamps and controlled to discharge the series capacitor and increase voltage, thereby providing an adequate starting potential.
Methods and circuits for facilitating starting and flickerless operation of discharge lamps are disclosed in U.S. Pat. No. 4,260,932 to V. Johnson. The V. Johnson invention uses a voltage increasing capacitor arrangement to provide increased starting potential.